1. Field of the Invention
This invention relates to improvements in methods and circuitry for converting digital numbers to analog output signals, and more particularly to improvements in methods and circuitry for converting a signed digital number to an analog output signal while maintaining an output offset voltage typically equal to the input offset voltage of only one operational amplifier to produce a precise linear output for both positive and negative digital input values.
2. Description of the Prior Art
This invention pertains to digital to analog (D/A) conversion in which an input number is a binary-coded or two's--complement digital number which is considered to be "bipolar", representing either a positive or negative number with the most significant bit (MSB) of the digital word being the "sign" bit. An analog ground, or zero voltage reference signal, is usually provided by the user external to the digital to analog converter, and the internal conversion in the D/A device may be generated based on a different reference voltage level. Therefore, it is often necessary to convert the output of the D/A converter to the reference provided by the user. This unfortunately entails the generation of offset voltages created by level-shifting and amplification which are difficult to minimize, and therefore creates an undesirable error in the output voltage for a given input number. This is unsatisfactory because precision is usually important in D/A conversion.
Previously, techniques have been developed which employ the generation of positive and negative currents which are fed to a single operational amplifier connected as a current to voltage converter. However, the accuracy (symmetry across the reference voltage level for given positive and negative input codes of equal magnitude) has been unsatisfactory due to the difficulty of accurately mirroring positive into negative current. Such techniques generally require trimming, which is a complicated and expensive process, especially for integrated semiconductor devices.
Another technique which has been used is a "unipolar" conversion in which currents are not mirrored into negative currents, but rather are sent to the current to voltage converter in their original state. A gain and level shifting stage then used to convert the level to form a "bipolar" output wherein both positive and negative output voltages (with respect to the reference) may occur. However, this technique creates undesirable offsets generated by the gain stage and the common mode rejection of the level shifting stage.
What is needed is a way to minimize the output offset voltage of the operational amplifier while accurately matching the positive and negative amplification of digital numbers.